Patent GB-822140-A (GB)
Title: Process for the production of unsaturated carboxylic acids
IPC: C07C57/055
Filed: 1958-02-28
Abstract: Catalysts for the oxidation of unsaturated aldehydes to unsaturated carboxylic acids comprise phosphomolybdic acid, which may be dehydrated, or salts thereof, if desired in conjunction with a support such as silica, titania or zirconium oxide. Specified salts are those of tin, bismuth and antimony.  The final catalyst generally contains at least 10% by weight of phosphomolybdic compound.  It is preferred that their surface area be not too great, suitably 1-250 sq. metres per gram.  They may be subjected to heat treatment after preparation.  The phosphomolybdic compound may be incorporated on the carrier by impregnation, or preferably, co-gelation.  It is stated that regeneration may be effected but no details are given.  In examples, catalysts are prepared as follows: (1) calcined silica gel is impregnated with aqueous phosphomolybdic acid, dried and roasted; (2) precipitated stannous hydroxide is added to a colloidal silica solution containing phosphoric and molybdic acids and the resulting stannous phosphomolybdate/silica gel dried and roasted; (3) a bismuth phosphomolybdate/silica gel is similarly prepared from bismuth nitrate solution; (4) and (5) a solution of molybdenum trioxide in phosphoric acid is added to silica sol and the mixture dried and roasted.ALSO:Unsaturated carboxylic acids are prepared by reacting unsaturated aldehydes with oxygen, which may be derived, at least in part from the catalyst, in the vapour phase in the presence of a catalyst comprising phosphomolybdic acid, which may be dehydrated, or a salt thereof.  The aldehydes may be aliphatic, alicyclic or aromatic-substituted aldehydes and may be substituted by, for example, hydroxy, ether, carboxylic acid, carboxylic ester, keto, nitro or halogen groups.  Many are specified.  Water in the vapour phase may be added to the reaction zone, e.g. up to 12 mols. per mol. of aldehyde.  The catalyst may be used with or without a support such as silica, possibly containing alumina, titania or zirconium oxide. Preferably the catalyst comprises at least 10% by weight of the phosphomolybdic compound.  It is preferred that the surface area of the catalyst be not too great, suitably 1-250 square metres per gram. The phosphomolybdic acid or salt thereof may be incorporated on the support by impregnation or, preferably, cogelation.  The prepared catalyst may be heat-treated.  Specified methods of making free phosphomolybdic acids are: (1) by mixing phosphoric and molybdic acid; (2) by double decomposition of salts; (3) by extraction with ether from acidified aqueous solutions; and (4) by ion exchange from phosphomolybdate salts.  The salts may be made in solution generally after acidifying and heating theoretical quantities of the reactants. Specified salts are bismuth, tin and antimony phosphomolybdates.  An excess of metal cation may be present. Air is the preferred source of oxygen.  0.5 to 50 mols. of oxygen may be used per mol. of aldehyde. Suitable reaction temperatures are from 500 DEG  to 900 DEG  F. Atmospheric pressure is generally used. Reactant/catalyst contact times of 1 to 150 seconds are operable. The process may be operated intermittently or continuously using a fixed or fluidized catalyst. The reactor temperature must be regulated to control the exothermic reaction.  The unsaturated acid may be recovered from the reactor effluent gases by conventional methods such as condensation, compression followed by expansion or scrubbing with water or other solvent.  The scrubbing solution may contain a polymerization inhibitor such as hydroquinone.  In examples air is used to oxidize acrolein to acrylic acid and methacrolein to methacrylic acid in the presence of water vapour, the acids being recovered by water scrubbing; catalysts are prepared as follows: (1) calcined silica gel is impregnated with aqueous phosphomolybdic acid, dried and roasted; (2) precipitated stannous hydroxide is added to a colloidal silica solution containing phosphoric and molybdic acids and the stannous phosphomolybdate/silica gel dried and roasted; (3) a bismuth phosphomolybdate/silica catalyst is prepared similarly to (2) using bismuth nitrate solution; (4) and (5) a solution of molybdenum trioxide in phosphoric acid is added to silica sol and the mixture dried and roasted.